Organic anion transporters (OATs) mediate the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. OATs are mainly expressed in the kidney, liver, brain and placenta. OAT dysfunction in these organs significantly contributes to the renal, hepatic, neurological and fetal toxicity and disease. Our long-term goal is to define the molecular mechanisms underlying drug disposition through the OAT pathway. During the previous grant period, significant progress and productivity have been achieved, and the new findings from this period led to the establishment of a fine-tuned research plan and strategy in this competing renewal. We propose to test the novel hypothesis that Nedd4-2, an ubiquitin ligase, serves as a central convergence point/switch for protein kinase-regulated OAT1 activity, and therefore for transducing diverse physiological stimuli to OAT1-mediated drug transport. Three Specific Aims are outlined. In Specific Aim I, we will map protein kinase-specific phosphorylation sites on Nedd4-2, and examine whether phosphorylation of Nedd4-2 is the mechanism by which diverse protein kinases regulate OAT1 transport activity. In Specific Aim II, we will identify the specific domains/amino acid residues in OAT1 and Nedd4-2, critical for the interaction between these two proteins. In Specific Aim III, we will evaluate the physiological role of Nedd4-2 in OAT1-mediated drug transport. Combined approaches of biochemistry and molecular biology will be employed for the proposed studies in cultured cells, and in kidney slices from normal and Nedd4-2 knockout mice. Understanding the role of dynamic phosphorylation of Nedd4-2 in the regulation of OATs, a novel focus in drug transport field, will have significant impact on the future design of strategies aimed at maximizing therapeutic efficacy and minimizing toxicity, and will permit insight into the molecular, cellular, and clinical bases of renal, hepatic, neurologica and fetal toxicity and disease.